In the past, numerous different approaches have been used for singulating or dicing a semiconductor wafer, the process of dividing a semiconductor wafer into individual devices. The two most widely used methods at this time are sawing using a diamond saw blade and laser scribing using a focused laser beam to cut through the wafer. Neither method is ideal. Both result in a significant loss of material during the cutting process. As the size of semiconductor devices get smaller, the width of the line of lost material during the scribing process becomes comparable to the width of the device. If the width of the line of material lost during the scribing process could be made significantly smaller, many more devices could be made on each wafer, resulting in a large savings in the cost of fabricating the devices.
Another method of wafer dicing, scribe-and-break, uses a tool with a diamond point or edge to create a line of stress between the devices to be separated and then another apparatus breaks the wafer along this line.
All three of these methods can cause damage along the cut edges of the devices that can result in rejected devices during visual inspection and in some cases cracking that can cause device failure in the field.
Since the invention of plasma and reactive ion etching in the 1970s, many individuals have proposed using these processes for wafer singulation. These processes potentially could decrease the material loss during the dicing process by etching very narrow scribe lines through the semiconductor wafer. In addition, since the etch process takes place at a microscopic level and involves no material melting or mechanical grinding, the edge of the semiconductor devices are not damaged by the process. In order for a plasma etching or a reactive ion etching process to be effective in wafer dicing, it would have to etch very deep, narrow trenches in the scribe streets of the semiconductor wafer and it would have to etch at a very fast etch rate to be economically attractive. These two conditions have been achieved in the last several years by the approach disclosed in Teixeira et al. U.S. Pat. No. 6,417,013 building on the work disclosed in Laermer et al. U.S. Pat. No. 5,501,893. The single issue that remains to be resolved is a cost effective method of separating the back metal that remains in the scribe streets after the etch process is completed.
Semiconductor wafers usually have back metal comprising one or more metal layers applied to the back of the wafer during fabrication to provide ohmic contact and/or ease of die attach during packaging of the devices. These layers of metal are not readily etched using dry etch processes. This invention teaches a method and apparatus to effectively separate these metal layers in the semiconductor wafer scribe streets.
It is current practice to mount the semiconductor wafer to be diced or singulated on a plastic film that is stretched across a metal or plastic frame. When using a scribe-and-break process, after the process has been completed the diced wafer is examined to ensure that the back metal has been separated between all of the devices. If the metal is not separated between any devices, it is common practice to manually press a stylus against the backside of the plastic film causing the film to deform upward and stretch causing the back metal to separate in this region.